296 Action of the Genetic Material 



regenerate.) But the polytene cells have acquired the capacity of 

 synthesizing huge quantities of a single proteinic substance, serin in 

 the silk-gland cell, myosin in the muscle cell, and just protoplasm for 

 growth without division in others. What has happened in these nuclei 

 is just the opposite of intranuclear genetic differentiation. It is ac- 

 tually the stoppage of the genetic activity of the chromosomes which 

 is replaced by a purely functional activity, restricted to one synthetic 

 action. This is also intranuclear differentiation, though of a very spe- 

 cific type and not comparable to what is meant by intranuclear change 

 as a means of genie differentiation. 



It is remarkable what happens in this case to the chromosomes. 

 According to new work of Bauer and Beermann ( 1952 ) , Herskowitz 

 (1950), and Beermann (1952) on salivary chromosomes of Chirono- 

 mus and Drosophila, the chromonema of each chromatid (which 

 probably is already a micellar bundle) multiplies this structure im- 

 mensely by endochromosomal division. The result is in Drosophila 

 a calculated number of about 1,000 strands, which thus go down to 

 submicroscopic size (though recent electron microscopic work shows 

 only 16, i.e., 4 for the chromatid). In Chironomus these innumerable 

 strands can be seen splitting off in the so-called Balbiani rings and 

 again split down to ultramicroscopic size and an estimated 16,000n 

 constitution. Hand in hand with this multiplication goes the increase 

 in DNA, proportional to the number of strands (in potencies of two), 

 as well as increase in cell size of growth without division ( see fig. 3Z? ) . 



From the point of view of cytology — perhaps also of function — 

 it might not seem correct to consider both the endomitotic polyploidy 

 as found in Gerris, silkworm glands, and so on, and the so-called poly- 

 teny of dipteran somatic chromosomes to be the same thing. In the 

 first case entire chromosomes have doubled, quadrupled, and so on, 

 and the individual chromosome groups may or may not remain sepa- 

 rate. Frequently, only the chromocenters and nucleoli indicate the 

 endomitotic groups, the chromosomes themselves being invisible in the 

 resting nuclei. The so-called polytene chromosomes of Diptera have a 

 completely different origin. The four synapsed chromatids retain their 

 individuality, though it is only occasionally visible (see Goldschmidt 

 and Kodani, 1942). Within these chromatids the micellar bundle of 

 the chromonema multiplies immensely; nobody knows whether this 

 happens by division into 2-4-8, and so on, though a 16 group 

 (=4x4) has been claimed in Drosophila and also a DNA quantity 

 in potencies of 2, as discussed and illustrated before, so that a bundle 

 of fibrils results which is an enlarged replica of the original micellar 



